Light emitting diodes, LEDs, usually have a relatively high light output with a high efficiency and small dimensions. LEDs can emit light with a light spectrum corresponding to a visible range or to an infrared range or to other non-visible frequency ranges.
LEDs can be used in backlight systems of displays of television or monitor systems. With LEDs it is possible to achieve a more evenly distributed lighting system than with a conventional neon light for example.
Controlling of the brightness of a LED can be done by varying the value of a current through the LED. This can lead to a variation of the spectral color of the LED. Another way of controlling LEDs is the use of pulse width modulated, PWM signals. In this case a mean current through the LED primarily has the same value.
FIG. 6 shows an embodiment of a conventional control circuit for controlling several strings STR1, STR2, STR5 of LEDs. Each string comprises a LED driving circuit VS1, VS2, VS5, for example a voltage source, and a switch S1, S2, S5. The brightness of the LED is controlled by switching on or off the strings STR1, STR2, STR5 depending on a pulse width modulated switching signal generated by a LED string switching control CCPWM. When switching the switches S1, S2, S5, a current flow for the LED strings STR1, STR2, STR5 is completely switched on or off. This leads to high current peaks or spurs which cause electromagnetic interference, EMI.
When using a pulse width modulation, the switching signal in a backlight system is usually synchronized with signals derived from a video signal, for example horizontal or vertical synchronization signals as HSYNC or VSYNC. This can lead to optical interference between the switching signal and the synchronization signals. It is possible that the back lighted screen becomes dark on the same position or at the same time respectively when generating the video image from the video signal. For example, the lower half of the screen can be always darker than the upper half because of the synchronization.